Canadian health officials said on Monday they would stop offering AstraZeneca’s COVID-19 vaccine to people under age 55 and require a new analysis of the shot’s benefits and risks based on age and gender.

The moves follow reports from Europe of rare but serious blood clots, bleeding and in some cases death after vaccination, mainly in young women. No such cases have been reported in Canada, with about 307,000 AstraZeneca doses administered.

“We are pausing the use of AstraZeneca vaccine to adults under 55 years of age pending further risk benefit analysis,” Canada’s deputy chief public health officer Howard Njoo said at a media briefing.

Njoo later noted that Canada was taking this “prudent” approach because alternative vaccines are available. Most of Canada’s supply so far has come from Pfizer Inc and Moderna Inc.

The National Advisory Council of Immunization (NACI), an independent expert panel, said the rate that the clotting complication happens at is not yet clear. So far, 40% of people who have developed it have died, but that may fall as more cases are identified and treated early, it said.

“From what is known at this time, there is substantial uncertainty about the benefit of providing AstraZeneca COVID-19 vaccine to adults under 55 years of age,” the council in a written recommendation.

Older people face a greater risk of hospitalization and death from COVID-19, and the complication seems to be more rare in that age group, NACI said, so they can be offered the vaccine “with informed consent.”

Health Canada said in a statement it would add new terms and conditions to the vaccine’s authorizations, including “a requirement that the manufacturers conduct a detailed assessment of the benefits and risks of the vaccine by age and sex in the Canadian context.”

Health Canada said it had been in talks with AstraZeneca, and once it has the requested information, it “will determine if additional regulatory actions are necessary.”

It was not immediately clear how long the assessment might take.

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